Re: powerpc/powernv/pci-ioda: fix kdump with non-power-of-2 crashkernel=

[Jan Stancek <jstancek@redhat.com>]

On Fri, Sep 04, 2015 at 09:59:38AM -0700, Nishanth Aravamudan wrote:
> The 32-bit TCE table initialization relies on the DMA window having a
> size equal to a power of 2 (and checks for it explicitly). But
> crashkernel= has no constraint that requires a power-of-2 be specified.
> This causes the kdump kernel to fail to boot as none of the PCI devices
> (including the disk controller) are successfully initialized.
> 
> After this change, the PCI devices successfully set up the 32-bit TCE
> table and kdump succeeds.
> 
> Fixes: aca6913f5551 ("powerpc/powernv/ioda2: Introduce helpers to allocate TCE pages")
> Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
> Cc: stable@vger.kernel.org # 4.2
> ---
> 
> Michael, I did this as a follow-on patch to my previous one. If you'd
> rather I made a v3 of that patch with the two fixes combined, I can
> resend.
> 
> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
> index f1c74c28e564..73914f4bd1ab 100644
> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
> @@ -2084,6 +2084,12 @@ static long pnv_pci_ioda2_setup_default_config(struct pnv_ioda_pe *pe)
>  	 */
>  	const u64 window_size =
>  		min((u64)pe->table_group.tce32_size, memory_hotplug_max());
> +	/*
> +	 * crashkernel= specifies the kdump kernel's maximum memory at
> +	 * some offset and there is no guaranteed the result is a power
> +	 * of 2, which will cause errors later.
> +	 */
> +	window_size = __rounddown_pow_of_two(window_size);

Hi,

Just wondering if this won't hide potential alignment issues of
"table_group.tce32_size", that now trigger EINVAL down the road
and if it wouldn't be safer to round only "memory_hotplug_max()?

  const __u64 hotplug_max_p2 = __rounddown_pow_of_two(memory_hotplug_max());
  const __u64 window_size =
            min((u64)pe->table_group.tce32_size, hotplug_max_p2);

Regards,
Jan

>  
>  	rc = pnv_pci_ioda2_create_table(&pe->table_group, 0,
>  			IOMMU_PAGE_SHIFT_4K,